Bipyridine metal complexes for use as light-emitting material

ABSTRACT

The present invention relates to light emitting materials comprising novel ortho-metalated transition metal complexes [C^N] 2 M[N^N] comprising two orthometalated ligands (C^N ligands) and a neutral bidentate bipyridine ligand (N^N). Surprisingly, it has been found that when the metal binds both orthometalated chelate C^N ligands and a neutral bidentate bipyridine ligand (N^N), these ligands participate in the emission process, thus greatly improving the red emission efficiency of complexes [C^N] 2 M[N^N]. The present invention further relates to the use of such light emitting materials and an organic light emitting device comprising such light emitting material.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. national stage application under 35 U.S.C.§371 of International Application No. PCT/EP2008/056954 filed Jun. 5,2008, which claims priority to European Application No. 07109876.8 filedJun. 8, 2007, these applications being incorporated herein by referencein their entirety for all purposes.

TECHNICAL FIELD

The present invention relates to a light-emitting material, the use ofsaid material and a light-emitting device capable of convertingelectrical energy into light.

BACKGROUND ART

Today, various display devices have been under active study anddevelopment, particularly those based on electroluminescence (EL) fromorganic materials.

In contrast to photoluminescence (i.e., light emission from an activematerial due to optical absorption and relaxation by radioactive decayof excited state), electroluminescence (EL) is a non-thermal generationof light resulting from the application of an electric field to asubstrate. In the latter case, excitation is accomplished by therecombination of charge carriers of contrary signs (electrons and holes)injected into an organic semiconductor in the presence of an externalcircuit.

A simple prototype of an organic light-emitting diode (OLED), i.e., asingle layer OLED, is typically composed of a thin film of an activeorganic material, which is sandwiched between two electrodes, one ofwhich needs to be semitransparent in order to observe light emissionfrom the organic layer. Usually, an indium tin oxide (ITO)-coated glasssubstrate is used as an anode.

If an external voltage is applied to the two electrodes, then chargecarriers (i.e., holes) at the anode and electrons at the cathode areinjected to the organic layer beyond a specific threshold voltagedepending on the organic material applied. In the presence of anelectric field, charge carriers move through the active layer and arenon-radioactively discharged when they reach the oppositely chargedelectrode. However, if a hole and an electron encounter one anotherwhile drifting through the organic layer, then excited singlet(anti-symmetric) and triplet (symmetric) states (i.e., so-calledexcitons) are formed. Light is thus generated in the organic materialfrom the decay of molecular excited states (or excitons). For everythree triplet excitons that are formed by electrical excitation in anOLED, only one symmetric state (singlet) exciton is created.

Many organic materials exhibit fluorescence (i.e., luminescence from asymmetry-allowed process) from singlet excitons. Since this processoccurs between states of same symmetry, it may be very efficient. On thecontrary, if the symmetry of an exciton is different from the one of theground state, then the radioactive relaxation of the exciton isdisallowed and luminescence will be slow and inefficient. Because theground state is usually anti-symmetric, the decay from a triplet breakssymmetry. The process is thus disallowed and the efficiency of EL isvery low. Therefore, the energy contained in the triplet states ismostly wasted.

Luminescence from a symmetry-disallowed process is known asphosphorescence. Characteristically, phosphorescence may persist up toseveral seconds after excitation due to the low probability of thetransition, which is different from fluorescence that originates in therapid decay.

However, only a few organic materials have been identified, which showefficient room temperature phosphorescence from triplets.

Successful utilization of phosphorescent materials holds enormouspromises for organic electroluminescent devices. For example, oneadvantage of utilizing phosphorescent materials is that all excitons(formed by combining holes and electrons in an EL), which are (in part)triplet-based in phosphorescent devices, may participate in energytransfer and luminescence. This can be achieved either byphosphorescence emission itself or by using phosphorescent materials toimprove the efficiency of fluorescence process.

In each case, it is important that the light emitting material provideselectroluminescence emission in a relatively narrow band centered nearselected spectral regions, which correspond to one of the three primarycolours (i.e., red, green and blue). This is so that they may be used asa coloured layer in an OLED.

As a means for improving the properties of light-emitting devices, therehas been reported a green light-emitting device utilizing the emissionfrom ortho-metalated iridium complex. (Ir(ppy)3: tris-ortho-metalatedcomplex of iridium (III) with 2-phenylpyridine (ppy). Appl. phys. lett.1999, vol. 75, p. 4.

Thus, US 2005214576 (SERGEY LAMANSKY ET AL.) 29 Sep. 2005 disclosesemissive phosphorescent organometallic compounds useful in thefabrication of organic light emitting devices, which are exemplified bythe following: platinum(II)(2-phenylpyridinato-N,C^(2′))(acetylacetonate) [Pt(ppy)(acac)]; platinum(II)(2-(p-tolyppyridinato-N,C^(2′))(acetyl acetonate) [Pt(tpy)(acac)];platinum(II)(7,8-benzoquinolinato-N,C^(3′)) (acetyl acetonate)[Pt(bzq)(acac)]; platinum(II)(2-benzylpyrinato-N,C^(2′)) (acetylacetonate) [Pt(bzpy)(ocac)];platinum(II)(2-(2′-thienyl)pyridinato-N,C^(3′)) (acetyl acetonate)[Pt(thpy)(acac)];platinum(II)(2-(2′-(4′,5′-benzothienyl)pyridinato-N,C^(3′)) (acetylacetonate) [Pt(btp)(acac)];platinum(II)(2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)) (acetylacetonate) [Pt(4,6-F₂ ppy)(acac)];platinum(II)(2-(4′,5′-difluorophenyl)pyridinato-N,C^(2′)) (acetylacetonate) [Pt(4,5-F₂ ppy)(acac)]; andplatinum(II)(2-(4′,5′-difluorophenyl)pyridinato-N,C²) (2-picolinato)[Pt(4,5-F₂ ppy)(pico)].

WO 2005/117159 (CDT OXFORD LIMITED) 8 Dec. 2005 discloses a metalcomplex for emitting light represented by formula I, which is shownbelow:

-   M-L    wherein M is a metal, L is a ligand and L comprises Ar that is a    substituted or unsubstituted heteroaryl ring, which contains at    least one phosphorus atom. This suggests that L is preferably a    bidentate ligand such as bipyridyl.

WO 2005/117160 (CDT OXFORD LIMITED) 8 Dec. 2001 discloses a chargedmetal complex useful for light emitting devices. The charged metalcomplex may be fluorescent or phosphorescent, which contains metal M andcoordinate ligand L. Suitable metals M include lanthanide metals,d-block metals and metals forming fluorescent complexes. Further, ligandL may be monodentate, bidentate or tridentate.

SPROUSE, S., et al. Photophysical effects of metal-carbon a bonds inortho-metalated complexes of Ir(III) and Rh(III). J. Am. Chem. Soc.1984, vol. 106, p. 6647-6653. disclose dichloro-bridged dimmers of thetype [M(L)₂Cl]₂, wherein L is 2-phenylpyridine (ppy) orbenzo[h]quinoline (bzq) and M is Rh(III) or Ir(III). The above referenceteaches that the ortho-metalated ligands exhibit considerably higherspectroscopic effects and lower energy charge transfer transitionscompared to Rh(III) and Ir(III) complexes of 2,2′-bipyridine (bpy) and1,10-phenanthroline (phen).

SLINKER, Jason D., et al. Efficient yellow electroluminescence from asingle layer of a cyclometalated iridium complex. J. Am. Chem. Soc.2004, vol. 126, p. 2763-2767. disclose a charged iridium complex,[Ir(ppy)₂-(dtb-bpy)]⁺(PF₆)⁻, and its use as a multifunctionalcyclometalating ligands. The charged iridium complex contains threeligands, wherein two cyclometalating ligands (ppy: 2-phenylpyridine) arechosen to coordinate the iridium metal center to further increase theligand field splitting energy. Further, the third ligand,4,4′-di-tert-butyl-2,2′-dipyridyl (dtb-bpy), ensures redoxreversibility, decreases self-quenching and enhances devicecharacteristics.

LEPELTIER, Marc, et al. Synthesis, structure and photophysical andelectrochemical properties of cyclometallated iridium(III) complexeswith phenylated bipyridine ligands. Eur. J. Inorg. Chem. 2005, p.110-117. disclose a series of cationic diminoiridium(III) complexes,[Ir(ppy-N,C)₂(L-N,N)](PF₆)(Hppy=2-phenylpyridine, L=4,4′-

u₂dpbpy, 4,4′-Me₂dpbpy, 4,4′-Me₂pbpy, 4,4′-Me₂bpy), and theirphotophysical and electrochemical properties.

SLINKER, Jason D., et al. Green electroluminescence from an ioniciridium complex. Appl. phys. lett. 2005, vol. 86, p. 173506. disclosegreen fluorescence from an ionic iridium complex,[Ir(F-mppy)2(dtb-ppy)]⁺(PF6⁻), wherein F-mppy is2-(4′-fluorophenyl)-5-methylpyridine and dtb-bpy is4,4′-di-tert-butyl-2,2′-bipyridine.

EVANS, Rachel C., et al. Coordination complexes exhibitingroom-temperature phosphorescence: Evaluation of their suitability astriplet emitters in organic light emitting diodes. CoordinationChemistry review. 2006, vol. 150, p. 2093-2126. disclose severaliridium(III) complexes containing cyclometalated ligands such as

-   4-(4′-chlorophenyl)-6′-phenyl-2,2′-bipyridine (clpby),-   4′-(4-carboxyphenyl)-6′-phenyl-2,2′-bipyridine (cpbpy),-   4,4′-dibutyl-2-2′-bipyridine (dbbpy),-   4-(4-hydroxyphenyl)-6′-phenyl-2,2′-bipyridine (hpbpy) or-   4′-(4-tolyl)-6′-phenyl-2,2′-bipyridine and its derivatives.

However, since the above light-emitting materials of the prior art donot display pure colours, i.e., their emission bands, which aregenerally limited to green, are not centered near selected spectralregions (corresponding to one of the three primary colours—red, greenand blue), the range that they can be applied as OLED active compound isnarrow. It has thus been desired to develop light-emitting materials,which are capable of emitting light with other colours, especially inthe red region.

Efficient and long-lived red-light emitters with good colour coordinatesare a recognized current shortfall in the field of organicelectroluminescent devices.

DISCLOSURE OF INVENTION

It is thus an object of the present invention to provide a lightemitting material comprising an ortho-metalated complex with anancillary ligand, as described below.

Another object of the present invention is the use of said lightemitting material, as well as to provide an organic light emittingdevice comprising said light emitting material.

As mentioned above, the object of the present invention is to provide alight emitting material, which comprises the complex of formula (I):

wherein:

-   M represents a transition metal having an atomic number of at least    40, which is preferably in groups 8 to 12, more preferably Ir or Pt,    and most preferably Ir;-   E₁ represents an aromatic or heteroaromatic ring optionally    condensed with additional aromatic moieties or non aromatic cycles,    wherein said ring optionally has one or more substituents and    optionally forms a condensed structure with the ring comprising E₂,    and wherein said ring coordinates to metal M by using a sp²    hybridized carbon;-   E₂ represents a N-containing aromatic ring optionally condensed with    additional aromatic moieties or non aromatic cycles, wherein said    ring optionally has one or more substituents and optionally forms a    condensed structure with the ring comprising E₁, and wherein said    ring coordinates to metal M by using a sp² hybridized nitrogen;-   R′ is the same or different at each occurrence and is selected from    the group consisting of —H, —F, —Cl, —Br, —NO₂, —CN, a straight or    branched C₁₋₂₀ alkyl, a C₃₋₂₀ cyclic alkyl, a straight or branched    C₁₋₂₀ alkoxy, a C₁₋₂₀ dialkylamino, a C₄₋₁₄ aryl, a C₄₋₁₄ heteroaryl    that may be substituted by one or more non aromatic radicals,    wherein a plurality of substituents R′ (either on the same ring or    on two different rings) may collectively form an additional mono- or    polycyclic ring system (optionally aromatic);-   X₁, X₂, X₃, X₄, X₅ and X₆ are the same or different at each    occurrence and are independently selected from hydrogen, alkyl,    aryl, heteroaryl and alkyl, each of which may be substituted by at    least one substituent;-   A⁻ is a counter anion; and-   n₁, n₂, m₁ and m₂ are the same or different at each occurrence and    represent an integer from 0 to 4, wherein n₁+m₁=4 and n₂+m₂=4,    provided that n₁ and n₂ cannot be zero at the same time.

As specified above in formula (I), the two chelating monoanionic ligandsare bound to the metal through carbon and nitrogen atoms, and comprisesE₁ and E₂ moieties. Such ligands are generally denoted as orthometalatedligands (“C^N ligands”).

The chelating bidentate bipyridine ligand bound to the metal through twonitrogen atoms is generally denoted as ancillary ligand (“N^N ligand”).

Surprisingly, it has been found that when the neutral chelate ligand(N^N) (also referred to as ancillary ligand) comprises a 2,2′-bipyridinebearing conjugated ethylenically unsaturated substituents to therebypossess adequate electron-accepting properties, said ligandadvantageously participates in the emission process. That is, suchligand significantly shifts emission toward lower energies (red-shift)and substantially improves the emission efficiency of complexes[C^N]₂M[N^N] in the red region.

Further, through the chelate ligand (NAN) bearing conjugatedethylenically unsaturated substituents, it is possible to obtain highlyphosphorescent light emitting materials comprising [C^N]₂M[N^N]complexes of formula (I), which has a maximum emission between 650 nmand 750 nm, thus corresponding to a red emission.

Preferably, the light emitting material of the present inventioncomprises the complex of formula (II):

wherein:

-   X₁, X₂, X₃, X₄, X₅, X₆, R′, n₁, n₂, m₁, m₂ and A⁻ have the same    meanings as defined above;-   X is chosen from the group consisting of —CH═CH—, —CR═CH—, —CR═CR—,    N—H, N—R¹, O, S and Se, wherein X is preferably selected from    —CH═CH—, —CR═CH— or S;-   Y is chosen from the group consisting of —CH═CH—, —CR═CH—, —CR═CR—,    N—H, N—R¹, O, S and Se, wherein Y is preferably selected from    CH═CH—, —CR═CH— or S;-   R is the same or different at each occurrence and represents —F,    —Cl, —Br, —NO₂, —CN, a straight or branched C₁₋₂₀ alkyl, a C₃₋₂₀    cyclic alkyl, a straight or branched C₁₋₂₀ alkoxy, a C₁₋₂₀    dialkylamino, a vinyl that may be substituted by one or more    aromatic or non aromatic radicals, a C₄₋₁₄ aryl, a C₄₋₁₄ heteroaryl    that may be substituted by one or more non aromatic radicals,    wherein a plurality of substituents R (either on the same ring or on    two different rings) may collectively form an additional mono- or    polycyclic ring system (optionally aromatic);-   a is an integer from 0 to 4; and-   b is an integer from 0 to 4.

More preferably, the light emitting material of the present inventioncomprises the complex of formula (IIA):

wherein:

-   X₁, X₂, X₃, X₄, X₅, X₆, X, Y, R, R′, n₁, n₂, m₁, m₂, a and A⁻ have    the same meanings as defined above;-   X₇, X₈ and X₉ are the same or different at each occurrence and are    independently selected from hydrogen, alkyl, aryl, heteroaryl and    alkyl, each of which may be substituted by at least one substituent;-   R″ is the same or different at each occurrence and represents —F,    —Cl, —Br, —NO₂, —ON, a straight or branched C₁₋₂₀ alkyl, a C₃₋₂₀    cyclic alkyl, a straight or branched C₁₋₂₀ alkoxy, a C₁₋₂₀    dialkylamino, a C₄₋₁₄ aryl or a C₄₋₁₄ heteroaryl that may be    substituted by one or more non aromatic radicals, wherein a    plurality of substituents R″ (either on the same ring or on two    different rings) may collectively form an additional mono- or    polycyclic ring system (optionally aromatic);-   b is an integer from 0 to 3; and-   w is an integer from 1 and 4.

Among the complexes of the present invention, the preferred ones arethose wherein X₁, X₂, X₃, X₄, X₅, and X₆ are each independently selectedfrom hydrogen and unsubstituted or substituted aryl groups.

More preferred ones are those wherein: two of X₁, X₂, and X₃ arehydrogen while the remaining one is a benzoic acid radial; two of X₄,X₅, and X₆ are hydrogen while the remaining one is a benzoic acidradical; or two of X₇, X₈ and X₉ are hydrogen while the remaining one isa benzoic acid radical.

The complex of formula (III), which is shown below, produced excellentresults:

wherein A⁻ has the same meaning as above defined.

The complex of formula (III), which comprises 2-phenyl-N-pyridine (ppy)orthometalated ligands and a bidentate bipyridine (bpy) bearingconjugated ethylenically unsaturated substituents as ancillary ligand,is particularly advantageous for the present invention due to itsemission in the red region with high colour purity.

The synthesis of complex of formula (I) (i.e., metal complex comprisingtwo orthometalated ligands (C^N ligands) and a neutral bidentatebipyridine ligand (N^N)) can be accomplished by any known method. Thedetails of synthetic methods, which are suitable for preparing thecomplexes of formula (I), are amply disclosed in the followingliteratures: “Inorg. Chem.,” No. 30, pg. 1685 (1991); “Inorg. Chem.,”No. 27, pg. 3464 (1988); “Inorg. Chem.,” No. 33, pg. 545 (1994); “Inorg.Chem. Acta,” No. 181, pg. 245 (1991); “J. Organomet. Chem.,” No. 35, pg.293 (1987); and “J. Am. Chem. Soc.,” No. 107, pg. 1431 (1985).

Generally, according to the first embodiment of the present invention,the complexes that comply with formula (I) can be prepared according tothe following reaction scheme:

Acid forms of the orthometalated ligands (H—C^N) and ancillary ligands(N^N) are commercially available or can be easily synthesized by usingwell-known organic synthesis reaction pathways.

In particular, orthometalated ligands (H—C^N) can be prepared with goodto excellent yields by Suzuki coupling the substituted pyridine compoundwith corresponding arylboronic acids, as described in Olivier Lohse, etal (The Palladium Catalyzed Suzuki Coupling of 2- and 4-chloropyridines.Syn. Lett. 1999, No. 1, pgs. 15-18) and U.S. Pat. No. 6,670,645 (DU PONTDE NEMOURS) 30 Dec. 2003.

Synthetic methods, which are particularly adapted for preparing thefluorinated orthometalated ligands (H—C^N), are described in JP2003113164 A (MITSUBISHI MATERIALS CORP) 18 Apr. 2003 and JP 2003113163A (MITSUBISHI MATERIALS CORP) 18 Apr. 2003.

If the transition metal is iridium, then trihalogenated iridium (III)compounds such as IrCl₃.H₂O, hexahalogenated Iridium (III) compoundssuch as M°₃ IrX°₆ (wherein X° is a halogen (preferably Cl) and M° is analkaline metal (preferably K)) and hexahalogenated iridium (IV)compounds such as M°₂ IrX°₆ (wherein X° is a halogen (preferably Cl) andM° is an alkaline metal (preferably K)) (“Ir halogenated precursors”)can be used as starting materials to synthesize the complexes of formula(I).

[C^N]₂Ir(μ-X°)₂Ir[C^N]₂ complexes (compound VI, wherein M=Ir), whereinX° is a halogen (preferably Cl), can be prepared from said Irhalogenated precursors and the appropriate orthometalated ligand byusing procedures already disclosed in literatures (S. Sprouse, K. A.King, P. J. Spellane, R. J. Watts, J. Am. Chem. Soc., 1984, 106,6647-6653; M. E. Thompson et al., Inorg. Chem., 2001, 40(7), 1704; M. E.Thompson et al., J. Am. Chem. Soc., 2001, 123(18), 4304-4312).

Preferably, reaction is carried out by using an excess of the neutralform of the orthometalated ligand (H—C^N). Further, high-boilingtemperature solvents are preferred.

The term “high-boiling temperature solvent” is intended to denote asolvent having a boiling point of at least 80° C., preferably at least85° C., and more preferably at least 90° C. For instance, suitablesolvents are methoxyethanol, ethoxyethanol, glycerol, dimethylformamide(DMF), N-methylpyrrolidone (NMP), dimethylsulfoxide (DMSO) and the like,wherein said solvents can be used as is or in admixture with water.

Optionally, reaction can be carried out in the presence of a suitableBrønsted base such as metal carbonates (especially potassium carbonate(K₂CO₃)), metal hydrides (especially sodium hydride (NaH)), metalethoxide or metal methoxide (especially NaOCH₃ and NaOC₂H₅),alkylammonium hydroxides (especially tetramethylammonium hydroxide) orimidazolium hydroxides.

A nucleophilic substitution at the metal atom with a suitable ligand(N^N), which is to form corresponding ([C^N]₂Ir[N^N])⁺A⁻ (formula I,wherein Me=Ir), is preferably carried out by roughly contacting astoichiometric amount of ligand N^N with bridged intermediate (VI) in asuitable solvent.

Polar aprotic solvents are generally preferred for this reaction. Asolvent, which produced particularly good results, is methylenedichloride (CH₂Cl₂).

The present invention is also directed to the use of a light emittingmaterial in the emissive layer of an organic light emitting device(OLED).

Furthermore, the present invention is directed to the use of a lightemitting material as dopant in a host layer, thus functioning as anemissive layer in an organic light emitting device.

If the light emitting material is used as dopant in a host layer, it isgenerally used in an amount of at least 1% wt, preferably at least 3%wt, and more preferably at least 5% wt with respect to the total weightof the host and the dopant. Further, it is generally used in an amountof at most 25% wt, preferably at most 20% wt, and more preferably atmost 15% wt.

The present invention is also directed to an organic light emittingdevice (OLED) comprising an emissive layer (EML), wherein said emissivelayer comprises the light emitting material described above. The OLEDcan optionally comprise a host material (wherein the light emittingmaterial is preferably present as a dopant), wherein said host materialis adapted to luminesce when a voltage is applied across the devicestructure.

The OLED generally comprises:

-   a glass substrate;-   a generally transparent anode such as an indium-tin oxide (ITO)    anode;-   a hole transporting layer (HTL);-   an emissive layer (EML);-   an electron transporting layer (ETL);-   a generally metallic cathode such as an Al layer.

For a hole conducting emissive layer, one may have an exciton blockinglayer, notably a hole blocking layer (HBL) between the emissive layerand the electron transporting layer. For an electron conducting emissivelayer, one may have an exciton blocking layer, notably an electronblocking layer (EBL) between the emissive layer and the holetransporting layer. The emissive layer may be equal to the holetransporting layer (in which case the exciton blocking layer is near orat the anode) or the electron transporting layer (in which case theexciton blocking layer is near or at the cathode).

The emissive layer may be formed with a host material, wherein the lightemitting material resides as a guest. Alternatively, the emissive layermay essentially comprise the light emitting material itself. In theformer case, the host material may be a hole-transporting materialselected from the group consisting of substituted tri-aryl amines.Preferably, the emissive layer is formed with a host material, whereinthe light emitting material resides as a guest. The host material may bean electron-transporting material selected from the group consisting ofmetal quinoxolates (e.g., aluminum quinolate (Alq₃), lithium quinolate(Liq), oxadiazoles and triazoles. An example of the host material is4,4′-N,N′-dicarbazole-biphenyl [“CBP”], which can be characterized bythe following formula:

CBP

Optionally, the emissive layer may also contain a polarization molecule,which is present as a dopant in said host material and having a dipolemoment, that generally affects the wavelength of light emitted when saidlight emitting material used as a dopant luminesces.

A layer formed from an electron transporting material is used totransport electrons into the emissive layer comprising the lightemitting material and the optional host material. The electrontransporting material may be an electron-transporting matrix selectedfrom the group consisting of metal quinoxolates (e.g., Alq₃ and Liq),oxadiazoles and triazoles. An example of an electron transportingmaterial is tris-(8-hydroxyquinoline)aluminum of formula [“Alq₃”]:

A layer formed from a hole transporting material is used to transportholes into the emissive layer comprising the light emitting material andthe optional host material. An example of a hole transporting materialis 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl [“α-NPD”].

The use of an exciton blocking layer (“barrier layer”) to confineexcitons within the luminescent layer (“luminescent zone”) is greatlypreferred. For a hole-transporting host, the blocking layer may beplaced between the emissive layer and the electron transport layer. Anexample of a material for such a barrier layer is2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (also referred to asbathocuproine or “BCP”), which has the following formula:

The OLED preferably has a multilayer structure (as depicted in FIG. 1),wherein: 1 is a glass substrate; 2 is an ITO layer; 3 is a HTL layercomprising α-NPD; 4 is an EML comprising CBP as host material and thelight emitting material as dopant in an amount of about 8% wt withrespect to the total weight of host plus dopant; 5 is a HBL comprisingBCP; 6 is an ETL comprising Alq₃; and 7 is an Al layer cathode.

EXAMPLES Synthesis of 2-phenyl-4-(2,5-dimethoxystyryl)pyridine

a)

uOK, DMF, rt

To a mixture of 2-phenyl-4-methylpyridine hydrochloride (1 g, 4.9 mmol)and 2,5-dimethoxybenzaldehyde (1.2 g, 7.3 mmol) in anhydrous DMF (40ml), solid t-BuOK (2 g, 18 mmol) was added. The resulting mixture wasstirred overnight at 80° C. under nitrogen. After evaporating DMF, Et₂Owas added thereto and the precipitate was filtered and washed withwater. The solid was purified by column chromatography (SiO₂,CH₂Cl₂/MeOH, 99/1) to afford 0.6 g (39%) of the desired compound as ayellow solid.

Synthesis of [(2-phenyl-4-(2,5-dimethoxystyryl)pyridine)₂IrCl]₂

b) IrCl₃.H₂O, EtOCH₂CH₂OH/H₂O, Δ.

1 equivalent of IrCl₃.3H₂O and 2.5 equivalents of2-phenyl-4-(2,5-dimethoxystyryl)pyridine were heated at 110° C. in amixture of 2-ethoxyethanol and water (3/1, v/v) overnight undernitrogen. After being cooled to room temperature, the resultingprecipitate was filtered off, successively washed with methanol (thanEt₂O) and finally dried to afford the desired dimer. Because of the lowsolubility of this compound, its ¹H-NMR was recorded in DMSO-d⁶ as its[C^N]₂ Ir(Cl)(DMSO) derivative.

¹H-NMR (DMSO-d⁶, 298K, 200 MHz, δ ppm) 3.80 (s, 12H), 3.88 (s, 12H),5.85 (d, J=7 Hz, 1H), 6.31 (d, J=7 Hz, 1H), 6.70-7.90 (m, 42H), 8.26 (s,1H), 8.32 (s, 1H), 9.45 (d, J=7 Hz, 1H), 9.75 (d, J=7 Hz, 1H).

Synthesis of[(2-phenyl-4-(2,5-dimethoxystyryl)pyridine)₂Ir(4,4′-dicarboxylic acid2,2′-bipyridine] [Comparative Complex (VII)]

[(2-phenyl-4-(2,5-dimethoxystyryl)pyridine)₂IrCl]₂ (122 mg, 0.071 mmol),4,4′-dicarboxylic acid 2,2′-bipyridine (40 mg, 0.164 mmol) andtetrabutylammoniumhydroxide 30 hydrate (261 mg, 0.326 mmol) wererefluxed in CH₂Cl₂ (100 ml) for 6 hours under argon. The resultingorange solution was concentrated to 5 mL and was crystallized by slowdiffusion of ethanol. The light yellow precipitate was filtered, washedwith Et₂O and air dried to afford 50 mg of the desired complex (yield:33%).

Synthesis of[(2-phenyl-4-(2,5-dimethoxystyryl)pyridine)₂Ir(4,4′-dicarboxylic acidstyryl)-2,2′-bipyridine] [Complex (III)]

[(2-phenyl-4-(2,5-dimethoxystyryl)pyridine)₂IrCl]₂ (83 mg, 0.048 mmol),(4,4′-paradicarboxylic acid styryl)-2,2′-bipyridine (44 mg, 0.095 mmol)and tetrabutylammoniumhydroxide 30 hydrate (194 mg, 0.242 mmol) wererefluxed in DMF (30 ml) for 8 hours under argon. The resulting solutionwas evaporated to dryness and the resulting solid was recrystallizedfrom methanol to afford 100 mg of the desired complex (yield: 80%).

The emission spectrum of complex (III) shows its maxima at around 710 nm(corresponding to red emission), with an emission intensity largelyexceeding that of reference complex (VII) and a red-shift of roughly 20nm with respect to reference complex (VII), thus enabling pure red coloremission to be obtained.

1. A complex of formula (I):

wherein: M represents a transition metal having an atomic number of atleast 40; E₁ represents an aromatic or heteroaromatic ring optionallycondensed with additional aromatic moieties or non aromatic cycles,wherein said ring optionally has one or more substituents and optionallyforms a condensed structure with the ring comprising E₂, and whereinsaid ring coordinates to metal M by using a Sp² hybridized carbon; E₂represents a N-containing aromatic ring optionally condensed withadditional aromatic moieties or non aromatic cycles, wherein said ringoptionally has one or more substituents and optionally forms a condensedstructure with the ring comprising E₁, and wherein said ring coordinatesto metal M by using a Sp² hybridized nitrogen; R′ is the same ordifferent at each occurrence and is selected from the group consistingof —H, —F, —Cl, —Br, —NO₂, —CN, a straight or branched C₁₋₂₀ alkyl, aC₃₋₂₀ cyclic alkyl, a straight or branched C₁₋₂₀ alkoxy, a C₁₋₂₀dialkylamino, a C₄₋₁₄ aryl, and a C₄₋₁₄ heteroaryl that is optionallysubstituted by one or more non aromatic radicals, wherein optionally aplurality of substituents R′, either on the same ring or on twodifferent rings, collectively form an additional mono- or polycyclicring system, optionally aromatic; X₁, X₂, X₃, X₄, X₅ and X₆ are the sameor different at each occurrence and are independently selected from thegroup consisting of hydrogen, alkyl, aryl, heteroaryl and alkyl, each ofwhich is optionally substituted by at least one substituent; A⁻ is acounter anion; and n₁, n₂, m₁ and m₂ are the same or different at eachoccurrence and represent an integer from 0 to 4, wherein n₁+m₁=4 andn₂+m₂=4, provided that n₁ and n₂ are not zero at the same time.
 2. Thecomplex of claim 1, represented by formula (II):

wherein: X₁, X₂, X₃, X₄, X₅, X₆, R′, n₁, n₂, m₁, m₂ and A⁻ have the samemeanings as defined in claim 1; X is selected from the group consistingof —CH═CH—, —CR═CH—, —CR═CR—, N—H, N—R¹, O, S, and Se; Y is selectedfrom the group consisting of —CH═CH—, —CR═CH—, —CR═CR—, N—H, N—R¹, O, S,and Se; R is the same or different at each occurrence and represents —F,—Cl, —Br, —NO₂, —CN, a straight or branched C₁₋₂₀ alkyl, a C₃₋₂₀ cyclicalkyl, a straight or branched C₁₋₂₀ alkoxy, a C₁₋₂₀ dialkylamino, avinyl that is optionally substituted by one or more aromatic or nonaromatic radicals, a C₄₋₁₄ aryl, or a C₄₋₁₄ heteroaryl that isoptionally substituted by one or more non aromatic radicals, whereinoptionally a plurality of substituents R, either on the same ring or ontwo different rings, collectively form an additional mono- or polycyclicring system optionally aromatic; a is an integer from 0 to 4; and b isan integer from 0 to
 4. 3. The complex of claim 2, represented byformula (IIA):

wherein: X₁, X₂, X₃, X₄, X₅, X₆, X, Y, R, R′, n₁, n₂, m₁, m₂, a and A⁻have the same meanings as defined in claim 2; X₇, X₈ and X₉ are the sameor different at each occurrence and are independently selected from thegroup consisting of hydrogen, alkyl, aryl, heteroaryl and alkyl, each ofwhich is optionally substituted by at least one substituent; R″ is thesame or different at each occurrence and represents —F, —Cl, —Br, —NO₂,—CN, a straight or branched C₁₋₂₀ alkyl, a C₃₋₂₀ cyclic alkyl, astraight or branched C₁₋₂₀ alkoxy, a C₁₋₂₀ dialkylamino, a C₄₋₁₄ aryl ora C₄₋₁₄ heteroaryl that is optionally substituted by one or more nonaromatic radicals, wherein optionally a plurality of substituents R″,either on the same ring or on two different rings, collectively form anadditional mono- or polycyclic ring system optionally aromatic; b is aninteger from 0 to 3; and w is an integer from 1 and
 4. 4. The complexaccording to claim 1, wherein X₁, X₂, X₃, X₄, X₅, and X₆ are eachindependently selected from the group consisting of hydrogen,unsubstituted aryl groups and substituted aryl groups.
 5. The complexaccording to claim 4, wherein two of X₁, X₂, and X₃ are hydrogen and theremainder is a benzoic acid radical.
 6. The complex according to claim4, wherein two of X₄, X₅, and X₆ are hydrogen and the remainder is abenzoic acid radical.
 7. The complex according to claim 3, wherein twoof X₇, X₈ and X₉ are hydrogen and the remainder is a benzoic acidradical.
 8. The complex according to claim 7, represented by formula(III):


9. A light emitting material comprising the complex according toclaim
 1. 10. A method for forming an emissive layer of an organic lightemitting device, comprising using the light emitting material accordingto claim 9 in the emissive layer.
 11. A method for forming an emissivelayer of an organic light emitting device, comprising using the lightemitting material according to claim 9 as dopant in a host layer,thereby functioning as the emissive layer in the organic light emittingdevice.
 12. An organic light emitting device (OLED) comprising anemissive layer (EML), wherein said emissive layer comprises the lightemitting material according to claim 9, and optionally a host material.13. A display device comprising the organic light emitting deviceaccording to claim 12.